A wire bonder is normally used for wiring or electrically contacting semiconductor components once they have been mounted on a carrier substrate. FIG. 1 schematically illustrates such a wire bonder. With the aid of a piezo actuator A, an axial ultrasonic wave having a frequency of approximately 100 kHz is generated in the wire bonder. An ultrasonic horn H transmits this ultrasonic wave to a capillary tube K, which thereby likewise vibrates at an amplitude of typically 1-10 μm mainly in axial direction Y of ultrasonic horn H (contactless state). Usual diameters of capillary tube K lie in a range between 60 μm and 150 μm. Capillary tube K, frequently made of ceramic material, has a small duct (not shown in the figure), through which the bonding wire is guided, which emerges from the tip of capillary tube K. The so-called bond head, which is made up of piezo actuator A, ultrasonic horn H, and capillary tube K, is often mounted on a slide unit V inside the wire bonder. Slide unit V is able to move the bond head in all three spatial directions X, Y, Z and thereby bring capillary tube K or the bonding wire to the individual sampling points.
The capillary tubes wear out with use, which is why they must be replaced on a regular basis. Because the different capillary tubes frequently have small mechanical tolerances, the capillary tube often has a slightly different vibrational amplitude after the exchange, and their clamping on the ultrasonic horn is not fully reproducible either. However, a different vibrational amplitude leads to fluctuating bond qualities in wire bonding. The time-consuming optimization of the bonding process using a first capillary tube that normally takes place can therefore be transferred to the subsequently used capillary tubes of the same type only to a very limited extent. In practice, this usually means that the probability of producing poor bond contacts, which constitute a high malfunction risk for the corresponding electronic device, is not negligible.
After each exchange of the capillary tube, the vibrational amplitude therefore is usually measured anew in the contactless state with the aid of a suitable device, and then brought back to the optimized value again by adapting the supply voltage of the piezo actuator. This makes it possible to increase the bond quality significantly.
To measure the vibrational amplitude of capillary tubes of a wire bonder, devices are known in which the capillary tube is situated between a light source and a detector system. In this case, the vibrational amplitude is able to be ascertained from the shading of a beam of light by the capillary tube. As far as devices based on a light barrier principle are concerned, reference is made to European Published Patent Application No. 1 340 582, U.S. Pat. No. 6,827,247 and Japanese Published Patent Application No. 10-209199, for example. In U.S. Pat. No. 6,827,247, the two edges of the capillary tube are sampled simultaneously and the vibrational amplitude ascertained from such sampling; in the devices described in European Published Patent Application No. 1 340 582 A1 and Japanese Published Patent Application No. 10-209199, only one edge of the capillary tube is sampled for this purpose.
In the devices described in the foregoing documents, the output signal generated at the detector system is modulated slightly by the ultrasonic movement of the capillary tube. Modulation amplitude ΔSAC of the output signal has an approximately linear relation to vibrational amplitude ΔyAC of the capillary tube, according to the following relationship:ΔSAC=η·ΔyAC  (eq. 1)where:ΔSAC:=the modulation amplitude of the output signalΔyAC:=the vibrational amplitude of the capillary tubeη:=the slope factor)
Slope factor η is a function of the luminous power of the light source used as well as the size of the light source image on the capillary tube, and the amplification factor of the evaluation electronics. Slope factor η, which should be as constant as possible, therefore has to be calibrated. This requirement in particular must also apply with regard to the working temperature, since the temperature conditions inside a wire bonder are usually not constant; instead, depending on the capacity utilization of the machine and the process, considerable temperature fluctuations may sometimes occur in a working temperature range of 20° C. to 60° C. In conventional approaches, such temperature fluctuations also result in fluctuations in the luminous power emitted by the light source. For instance, at an assumed typical temperature coefficient of the luminous power of a light source of −0.3%/K and a temperature rise of 40K, the emitted luminous power is reduced by 12%. The aforementioned slope factor η consequently changes by 12%, as well, which in turn leads to considerable errors in the determination of vibrational amplitude ΔyAC of the capillary tube. It is for this reason that conventional devices for measuring the vibrational amplitude in wire bonders provide only limited accuracy.
Another problem of conventional devices is the noise of the generated output signal. In principle, such noise should be as low as possible. Very low noise values are necessary to be able to ascertain vibrational amplitudes ΔyAC at the required high reproducibility. In the devices from the above-mentioned documents, the transmitted luminous power is measured with the aid of a photoelement in the detector system, which thus constitutes the output signal of the light barrier, which will then be amplified and digitized in addition. At the working point of the light barrier, with the capillary tube covering approximately one half of the light spot, small modulation amplitudes ΔSAC must be ascertained at a high signal offset level SDC that corresponds to this halfway coverage. The amplifier thus has to be configured accordingly, so that it is able to amplify the high signal offset level S. In this case, the amplification factor must be selected so that offset level SDC does not lead to a saturation of the amplifier. Because of the correspondingly lower amplification, the amplifier is unable to be used in a noise-optimized manner. The noise levels of the devices for measuring the vibrational amplitude of capillary tubes in wire bonders described in the above-mentioned documents are therefore relatively high.